JPS6142774B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a composite cast iron roll used as a work roll for hot-rolled materials, and more particularly to a strong cast iron composite roll whose outer shell material is made of high-alloy grain cast iron. As is well known, work rolls for hot rolled materials include:
Various conditions are required such as wear resistance, toughness, heat cracking resistance, and spalling resistance. It is difficult to fully satisfy these conditions with a single material, so recently composite rolls in which the outer shell layer and the inner layer are made of different materials are often used. Recently, high-alloy grain cast iron is often used as the outer shell material of such composite rolls, as it has extremely high hardness and excellent wear resistance. This high alloy grain cast iron is, for example, C3.3%, Si0.7
%, Mn0.6%, Ni4.5%, Cr1.8%, Mo0.4%, etc., and exhibits high hardness with shore hardness Hs80 or higher. However, as mentioned above, high-alloy grain cast iron is extremely hard and has a large amount of carbide crystallization, so it lacks toughness, so it is not suitable as an inner layer material for composite rolls with high-alloy grain cast iron as the outer shell material. is required. In particular, recently, hot rolling has been carried out under harsh conditions such as low-temperature rolling and high-speed rolling for the purpose of energy saving and cost reduction, and as a result, the rolls have outstanding flatness resistance and toughness. Therefore, as mentioned above, high strength and toughness are also required for the inner layer material of a composite roll whose outer shell material is made of high-alloy grain cast iron. By the way, as the inner layer material of a composite roll whose outer shell material is high-alloy grain cast iron, conventionally, normal cast iron with flake graphite or tough cast iron with small amounts of alloying elements such as Ni and Cr added thereto has been generally used. However, cast iron containing this type of flake graphite has extremely low mechanical strength, with a tensile strength of about 15 to 25 Kg/ ^mm2 and an elongation of less than 1%, and is therefore inadequate to meet recent demands for improved roll strength. It was enough. On the other hand, spheroidal graphite cast iron, which is made from spherical graphite, is also used as the inner layer material of some composite rolls. In this case, the inner layer material strength is a tensile strength of 40 to 50 kg/
Although it improves to about mm ² , there are various problems as follows. In other words, when the outer shell is made of high-alloy grain cast iron and the inner layer is made of spheroidal graphite cast iron, a layer in which a large amount of cementite precipitates or a layer of flaky graphite cast iron due to poor spheroidization of graphite is formed at the boundary between them, and as a result, the outer shell The strength of the boundary between the outer layer and the inner layer decreases significantly, which causes peeling of the outer shell layer during rolling. For example, according to experiments conducted by the present inventors, as shown in the microstructure photograph in FIG. It was confirmed that an abnormal graphite layer 3 similar to the above was formed, and the tensile strength of the abnormal graphite layer decreased to about 23 to 28 Kg/mm ² . In order to solve this problem, it is conceivable to cast an intermediate layer as a third phase between the outer shell of high-alloy grain cast iron and the inner layer of spheroidal graphite cast iron, but in this case, the roll There is a problem in that the manufacturing process becomes complicated and costs increase. Additionally, when spheroidal graphite cast iron is used as the inner layer material, dross generated from the molten spheroidal graphite cast iron during pouring may remain in the center of the loin and cause internal defects, so measures must be taken to float and remove the dross. . Furthermore, molten spheroidal graphite cast iron has a large volumetric shrinkage during solidification and tends to generate cavities, so a huge feeder is required to prevent defects. Therefore, when spheroidal graphite cast iron is used as the inner layer material,
There is a problem that the manufacturing process becomes complicated and the product yield also decreases. This invention was made in view of the above circumstances, and it is possible to sufficiently improve the strength of the inner layer material in a composite roll whose outer shell material is made of high-alloy grain cast iron than that of conventional ordinary cast iron or tough cast iron with flake graphite structure. At the same time, the material is made to be equivalent to that of spheroidal graphite cast iron, and at the same time, there is no material deterioration area at the boundary between the outer shell and the inner layer, unlike when spheroidal graphite cast iron is used as the inner layer material.Moreover, the manufacturing process is simple and the product is easy to use. It is an object of the present invention to provide a composite roll with good yield. In other words, the present inventors conducted various studies on the inner layer material when the outer shell material is high-alloy grain cast iron, and found that the inner layer material is vermicular cast iron with graphite shaped like a caterpillar (vermicular shape) between flaky and spherical. In addition, by regulating the sulfur content of the high-alloy grain cast iron used as the outer shell material to a low value,
It was discovered that the above-mentioned object can be achieved, and the present invention was completed. As mentioned above, vermicular cast iron has a graphite shape that is between flaky and spherical, and its strength is comparable to that of spheroidal graphite cast iron, with a tensile strength of 35 to 50 kg/
It is about ^mm2 . In addition, the castability of vermicular cast iron is as good as that of normal cast iron, and the occurrence of internal shrinkage cavities due to volume shrinkage during solidification is as small as that of normal cast iron. As shown in Figure A, the occurrence of shrinkage cavities in this case is much less than in the case of the spheroidal graphite iron casting shown in Figure 2C, and is comparable to that in the case of the ordinary cast iron casting shown in Figure 2B. In addition, the casting temperature of vermicular cast iron is 1350~
When used as the inner layer material of a composite roll whose outer shell is made of high-alloy grain cast iron, there is no deterioration of the graphite shape at the boundary between the outer shell and the inner layer. In addition, by suppressing the S content of the high-alloy grain cast iron used as the outer shell material, it is possible to prevent a region of reduced strength from occurring at the boundary. Therefore, by using vermicular cast iron as the inner layer material of a composite roll whose outer shell is made of high-alloy grain cast iron and by suppressing the S content of the high-alloy grain cast iron of the outer shell, peeling of the outer shell layer during rolling can be prevented. The inner layer can be strengthened without causing any danger, and since there are few internal shrinkage cavities, an excessively large feeder is not required, and a drop in casting yield can be avoided. Therefore, in the composite roll of the present invention, the outer shell is made of high-alloy grain cast iron, the S content of the high-alloy grain cast iron of the outer shell is regulated to 0.020% or less, and the inner layer material is C2.5~ 4.0%,
Si is within the range of 2.0 to 3.5% and carbon saturation Sc is 0.80
Contains C and Si so that it is ~1.20 and
Contains one or more of Mg0.010-0.025%, Ca0.002-0.005%, rare earth elements 0.010-0.020%, and further includes Mn1.5% or less, Ni1.5% or less, Cr0.8
% or less, Mo is 1.5% or less, and the balance is Fe and unavoidable impurities. The composite roll of the present invention will be explained in more detail below. As mentioned above, the composite roll of the present invention uses high-alloy grain cast iron as the outer shell material. As this high-alloy grain cast iron shell material, a material having a known composition other than S may be used. That is, usually C2.8~3.8%, Si0.8~1.8%, Mn0.3~1.0%,
High-alloy grain cast iron consisting of P0.3% or less, Cr0.5-2.0, Ni5.0% or less, and if necessary, _M0 of about 0.3% or less is used. In particular, in the case of this invention, S in the high alloy grain cast iron that is the outer shell material is
Regulated to 0.020% or less. The reason for this is as follows. In other words, when melting vermicular cast iron that becomes the inner shell material, one or more of Mg, Ca, and rare earth elements are added to the molten metal, and the graphite shape is made into a vermicular shape by its action. However, all of these elements have a strong affinity with S, so when the molten inner shell is injected, at the boundary with the molten outer shell, S in the molten outer shell, Mg in the molten inner shell,
Ca or rare earth elements combine to form sulfides, and the sulfides remain at the boundary between the outer shell and the inner layer and become nonmetallic inclusions, causing local strength reduction and forming boundaries during rolling. There is a risk that the outer shell may peel off from the part. On the other hand, if the S content in the high-alloy grain cast iron of the outer shell material is 0.02% or less, the amount of sulfide-based nonmetallic inclusions formed will decrease, and the shape will become fine, on the order of several microns. Therefore, the strength is not affected, and problems such as peeling of the outer shell from the boundary during rolling use do not occur. On the other hand, as mentioned above, the inner layer material is C2.5~4.0.
%, Si Adjust the amount of C and Si so that it is within the range of 2.0 to 3.5% and the carbon saturation Sc defined by C (%) / {4.23 - Si (%) / 3.2} is 0.80 to 1.20. and further Mg0.010~
0.025%, Ca0.002~0.005%, rare earth elements such as Ce
Contains 0.002 to 0.005% of one or more types, and Mn1.5
% or less, Ni 1.5% or less, Cr 0.8% or less, Mo 1.5% or less, and the balance is Fe and unavoidable impurities. The reason for limiting the components of the inner layer material is as follows. C, along with Si in cast iron, is a major element that determines the structure of cast iron, and the two perform functions that are closely related to each other. When the C content is less than 2.5%, the amount of graphite in the solidified vermicular cast iron is small, and the shape thereof is often flaky, resulting in a decrease in strength. On the other hand, when the C content exceeds 4.0%, the graphite shape becomes coarse and the molten metal composition shifts to the hypereutectic side. Therefore, when alloys such as Mg, Ca, and rare earth elements are added, the amount of vermicular graphite decreases. The amount of spheroidal graphite increases, resulting in a structure similar to that of spheroidal graphite cast iron, resulting in an increase in the amount of internal shrinkage cavities during solidification. Si is an element that has a large effect on the material quality of cast iron in relation to C, but if the Si content is less than 2.0, graphitization tends to be insufficient, and cementite precipitates and is mixed in, reducing the list price of strength, especially toughness. invite When the Si content exceeds 3.5%, the amount of graphite crystallized increases and the matrix structure becomes ferrite. Therefore, when the roll shaft core is formed, the hardness of the roll neck parts such as the drive part becomes low and the amount of wear decreases. It is not desirable because it increases. Furthermore, the content of C and Si is determined so that the carbon saturation Sc, which indicates the degree of eutecticism of cast iron, is within the range of 0.80 to 1.20, that is, the value of C (%) / {4.23-Si (%) / 3.2} regulate. Carbon saturation Sc in vermicular cast iron
As for the relationship between the strength and shrinkage cavities, the relationship shown in Fig. 3 has been obtained through experiments. However, in FIG. 3, curve 4 represents the amount of internal shrinkage cavities, and curve 5 represents the tensile strength. If the carbon saturation is less than 0.80, the amount of shrinkage cavities during solidification will be extremely low and internal integrity will be maintained, but the amount of flaky graphite will increase to 30% of the total graphite amount.
As a result, the mechanical properties, particularly the tensile strength, deteriorate, making it impossible to obtain high strength. On the other hand, if the carbon saturation exceeds 1.00, the graphite structure in cast iron will be a mixture of vermicular graphite and spheroidal graphite. Furthermore, if the carbon saturation degree Sc exceeds 1.20, the proportion of spheroidal graphite exceeds 30% and the proportion of vermicular graphite becomes lower than 70%, and if the proportion of spheroidal graphite increases in this way, the solidification form becomes spheroidal graphite cast iron. Although the mechanical strength is improved,
The amount of internal shrinkage cavities increases significantly, making it impossible to obtain a roll with good internal soundness. Therefore, in order to satisfy both high strength and internal soundness as an inner layer material of a composite roll, it is necessary to adjust the content of C and Si so that the carbon saturation degree is within the range of Sc from 0.80 to 1.20. There is. Mg, Ca, and rare earth elements are added to the molten cast iron before casting, usually in the form of an alloy with Fe or Si. It is used to crystallize. If these additions are not made, the graphite shape will not become vermicular, and the graphite will become notches, making it impossible to obtain high strength. However, since all of these have the ability to make graphite spheroidize, they are not added to the molten iron. If these elements remain in excess, spheroidization will proceed excessively and the proportion of vermicular graphite will decrease. Therefore, the residual amount of these elements after the addition treatment, that is, the content in cast iron, needs to be determined depending on the graphite nodularity of each element. That is, in the case of Mg
0.010-0.025%, 0.002-0.005% for Ca,
In the case of rare earth elements such as Ce, the content should be in the range of 0.010 to 0.020%. By setting it within such a range, it is possible to obtain vermicular cast iron in which 70% or more of the graphite in all the graphite grains in the cast iron is in a vermicular form. If the content is less than the above range, there will be a large amount of flaky graphite, making it impossible to improve the strength.On the other hand, if it exceeds the above range, the amount of spheroidal graphite will increase, causing expansion during solidification. The amount of internal shrinkage cavities increases. Note that Mg, Ca, and rare earth elements are 1 of these.
Seeds may be added singly, or two or more types may be added in combination. However, in the case of combined addition, it is desirable that the residual amount of each element satisfies the above-mentioned range and the total amount thereof is determined to be 0.025% or less. If the total amount exceeds 0.025%, the amount of spheroidal graphite increases as described above, and the amount of internal shrinkage cavities may increase. Furthermore, since the inner layer material also forms the driving part of the roll, its strength and wear resistance are high. In order to ensure hardness, it is preferable to add a small amount of Ni, Cr, Mo, and Mn to the inner layer material as in the case of ordinary cast iron, and the reason for limiting the upper limit is as follows. Ni is a strength-improving element, but if it exceeds 1.5%, the strength-improving effect is no longer recognized, and in cast iron, it promotes graphitization and becomes a ferrite base, making it soft, so the upper limit for Ni is 1.5%. did. Note that in order to effectively exhibit the effect of Ni addition, it is preferable to add 0.3% or more. Cr is a carbide stabilizing element, and when added in small amounts, the base structure becomes pearlite, increasing strength and hardness, and is effective in improving wear resistance. On the other hand, Cr
If it exceeds 0.8%, cementite will precipitate, resulting in a decrease in toughness and the risk of breakage when subjected to sudden impact. Therefore, Cr was set to 0.8% or less. Like Cr, Mo is also a carbide stabilizing element, and it contributes to improving strength when added in small amounts, but the effect does not change when the content exceeds 1.5%.
The following was made. Mn is an element that has a strong influence on the base structure, and is effective in improving the strength and hardness of the roll neck by turning the base structure into pearlite.
Since the degree of pearlitization does not change even if more than 1.5% of Mn is added, the Mn content was set to 1.5% or less. In addition
If Mn is less than 0.3, the above-mentioned effects cannot be sufficiently obtained, so it is preferable that Mn is 0.3% or more. Furthermore, cast iron usually contains P and S as unavoidable impurities, but in the vermicular cast iron used as the inner layer material of this invention, P is
It is preferable that S is 0.100% or less, and S is 0.030% or less. In other words, P has the effect of increasing the fluidity of molten cast iron, but when a large amount of P is added, Fe-C-P eutectic precipitates segregate at grain boundaries and cause a decrease in strength, so it has no effect on strength. It is preferable to limit the amount to 0.100% or less. S is added to make the graphite shape caterpillar-like.
In order not to lose the effects of Mg, Ca or rare earth elements, it is preferable to control their content.
That is, S has an extremely strong binding force with Mg, Ca, or rare earth elements, and when these elements are added to molten iron containing S, sulfides are immediately formed and the graphite shape control effect of these elements is lost.
Even in molten iron containing more than 0.030% S, it is possible to obtain caterpillar graphite by adding a large amount of the above elements, but a large amount of formed sulfides remain, leading to a decrease in strength. Therefore, the S content is 0.030
It is desirable to limit the amount to less than %. The method for manufacturing the composite roll of this invention is to fill a mold with molten high-alloy grain cast iron that will become the outer shell, wait for the outer shell to solidify, and then pour the unsolidified molten metal into vermicular cast iron that will become the inner layer. The so-called hollow casting method, in which the inner layer is solidified by replacing the molten metal, or the molten high-alloy grain cast iron, which will become the outer shell, is poured into a rotary mold to a predetermined thickness, and then the molten vermicular cast iron, which will become the inner layer, is injected to form the roll shaft core. Any method such as centrifugal casting may be used to form the part. The solidification temperature of the molten vermicular cast iron that forms the inner layer is almost the same as that of ordinary cast iron used for the inner layer of conventional composite rolls, and the fluidity of the molten metal is also almost the same, so the casting temperature of the molten vermicular cast iron that forms the inner layer There is no problem at about 1260 to 1350℃, which is the same as for conventional ordinary cast iron. In addition, in the case of centrifugal casting, there are two methods for injecting the molten metal that will form the inner layer: a method in which the entire amount is injected at once, and a method in which the molten metal is injected in two or more parts. Either can be used. Below, an example in which the present invention is applied to a hot-rolled post-finish work roll, and a comparative example of a composite roll using conventional ordinary cast iron as the inner layer material will be described. Example 1 The outer shell is made of high-alloy grain cast iron and the inner layer is made of vermicular cast iron, roll size φ750
A composite roll of mm×2300 mm was manufactured by centrifugal casting. The chemical composition of the high alloy grain cast iron of the outer shell is C3.27%, Si0.72%, Mm0.60%, P0.049%,
S0.013%, Ni4.34%, Cr1.79%, Mo0.36%, balance
It is made of Fe, and was particularly melted to have an S content of 0.020% or less. On the other hand, the molten metal of the vermicular cast iron that forms the inner shell is made of Fe-45%Si-25%Ca before casting.
A molten metal was treated by adding 1% of the alloy. The chemical composition of vermicular cast iron after treatment is C3.32%, Si2.53
%, Mn0.56%, P0.051%, S0.010%, Ni0.79%,
Cr0.22%, Mo0.10%, Ca0.0034%, balance Fe. The pouring temperature of the molten vermicular cast iron that forms the inner layer was 1330°C, and the entire amount of the molten metal was poured at one time. Comparative Example A composite roll similar to the above, having an outer shell made of high-alloy grain cast iron and an inner layer made of ordinary cast iron, was manufactured by centrifugal casting. The chemical composition of the high alloy grain cast iron of the outer shell is the same as in the example, and the chemical composition of the ordinary cast iron of the inner layer is C3.29%, Si1.22%, Mn0.46%,
P0.085%, S0.016%, Ni0.75%, Cr0.50%,
Mo0.14%, balance Fe. The casting temperature of the ordinary cast iron that forms the inner layer was 1335°C, and the entire amount of molten metal was poured in one go. The microstructure of the inner layer of the roll of Example 1 was
The microstructure of the inner layer of the roll of the comparative example is shown in FIG. As is clear from the comparison of these microstructure photographs, the structure of the inner layer of Example 1 of the present invention has an improved graphite shape and a notch shape compared to the structure of the inner layer using conventional ordinary cast iron. It is confirmed that the structure has a vermicular graphite structure that does not FIG. 6 shows a microstructure photograph of the boundary between the outer shell and the inner layer of the composite roll according to Example 1 of the present invention. In FIG. 6, reference numeral 6 indicates an outer shell made of high-alloy grain cast iron, 7 an inner layer made of vermicular cast iron, and 8 a boundary between the outer shell and the inner layer. As is clear from FIG. 6, there are no defects such as poor welding in the boundary 8, and as shown in FIG. A good structure with no discontinuity in material change from the outer shell to the inner layer can be obtained. Regarding the mechanical strength of the welded part (boundary part) between the outer shell and the inner layer, the tensile strength of the welded part of a comparative composite roll using conventional ordinary cast iron as the inner layer material is
The tensile strength of the welded part was 15 to 18 Kg/mm ² , whereas in the composite roll of Example 1 of the present invention using vermicular cast iron as the inner layer material, the tensile strength of the welded part was 28 to 32 Kg/mm 2 .
It was found that the strength of the welded part was doubled when the thickness of the welded part became approximately mm ² . Therefore, it is clear that the composite roll of the present invention can effectively prevent accidents such as cracking in the welded portion and peeling of the outer shell layer during rolling use. Furthermore, the tensile strength of the inner layer was 18 to 28 for the composite roll of the comparative example using ordinary cast iron as the inner layer material.
Kg/mm ² and Young's modulus remained at about 11,000 to 14,000 Kg/mm ² , whereas in the composite roll of Example 1 of this invention using vermicular cast iron as the inner layer material, the tensile strength of the inner layer was 41.1 to 44.4 Kg/mm. ² , Young's modulus
An excellent value of 17000Kg/ ^mm2 was obtained. The tensile strength of the inner layer of a composite roll whose inner layer material is spheroidal graphite cast iron is about 40 to 50 Kg/mm ² , therefore, the composite roll of this invention is compared with a composite roll whose inner layer material is spheroidal graphite cast iron. It is clear that the strength of the inner layer material is comparable to that of other materials. Example 2 As shown in No. 1 to No. 3 of Table 1, examples of adding Mg as an inner layer material, examples of adding rare earth elements (Ce), and examples of adding Mg and Ca at the same time were Table 2 shows the mechanical properties of similar composite rolls.

【table】

[Table] In the case of Mg addition, REM addition, or Mg+Ca addition in Example 2, as in Example 1, no abnormal tissue was formed at the boundary between the outer shell and the inner layer, and the mechanical properties were significantly improved. found. In addition, although the solid roll for plate rolling in which the roll core part is filled with the mixed inner layer material has been described in the example, it is also applicable to a composite sleeve-shaped hollow roll of the type that uses a separately manufactured shaft core part that is shrink-fitted. Of course, this invention can be applied. As is clear from the above description, the present invention provides a composite roll in which the outer shell is made of high-alloy grain cast iron, and the inner layer is made of vermicular cast iron having a caterpillar graphite structure instead of the conventional ordinary cast iron material.
And the S content of the high alloy grain cast iron of the outer shell is 0.020.
% or less. In such a composite roll of the present invention, the strength of the inner layer is extremely high, about twice that of the conventional case where ordinary cast iron is used for the inner layer, and the strength is localized at the boundary (welded part) between the outer shell and the inner layer. Since there is no abnormal tissue layer that deteriorates during rolling, problems such as cracks from the boundary and peeling of the outer shell layer do not occur during rolling use.
Moreover, since the amount of internal shrinkage cavities during solidification of the inner layer is small, the internal soundness is also good. In addition, in manufacturing the composite roll of the present invention, as mentioned above, the amount of internal shrinkage cavities is small, and special measures such as casting another third layer between the outer shell and the inner layer are used. Since there is no need to take any steps, we can use the same method as in the case of manufacturing composite rolls using conventional ordinary cast iron as the inner layer material.
By using the equipment, it is possible to easily obtain a composite roll with high strength, internal soundness, and durability. Therefore, according to the present invention, the usability of the roll under harsh conditions can be significantly improved compared to the conventional method, and the roll manufacturing cost does not particularly increase. Obtainable.

[Brief explanation of the drawing]

Figure 1 is a photograph of the microstructure near the boundary between the outer shell and inner layer of a composite roll that uses high-alloy grain cast iron as the outer shell material and spheroidal graphite cast iron as the inner layer material.
Figures A to C are cross-sectional photographs showing the state of shrinkage cavities in various spherical cast iron castings, Figure 3 is a correlation diagram showing the relationship between carbon saturation, tensile strength, and amount of shrinkage cavities in vermicular cast iron, and Figure 4 is A microstructure photograph of the inner layer of vermicular cast iron in the composite roll of the embodiment of this invention, FIG. 5 is a microstructure photograph of the inner layer of normal cast iron in the conventional composite roll, and FIG. This is a photograph of the microstructure near the boundary between the shell and the inner layer.

Claims (1)

[Claims] 1. In a composite roll whose outer shell is made of high-alloy grain cast iron, the S content of the high-alloy grain cast iron of the outer shell is
In addition to regulating the inner layer material to 0.020% (weight%, the same hereinafter) or less, the inner layer material must be within the range of C2.5 to 4.0% and Si2.0 to 3.5%, and C (%) / {4.23 - Si (%) / 3.2 } Contains C and Si such that the carbon saturation degree Sc specified by
Mn1.5% or less, Ni1.5% or less, Cr0.8% or less,
Contains Mo1.5% or less, and Mg0.010 to 0.025
%, Ca0.002~0.005%, rare earth elements 0.010~0.020
%, and the remainder is
A strong cast iron composite roll characterized by being made of vermicular cast iron containing Fe and inevitable impurities.